1. Field of the Invention
The present invention relates generally to the field of electric motors and seals for such motors. More particularly, the invention relates to a novel arrangement for maintaining lubricant, such as oil mist, within a bearing cavity to supply continuous lubrication to rotor bearings during operation of the motor, and for enhancing sealing of regions within which the rotor bearings are disposed.
2. Description of the Related Art
A wide variety of electric motors are available and are currently in use throughout a range of industrial applications. In general, such motors include a stator provided in a motor housing, and a rotor surrounded at least partially by the stator and supported for rotation within the housing. The stator and rotor may be mechanically and electrically configured in various manners, depending upon the application, the power available to drive the motor, and so forth. In general, however, electric power is applied to the stator and the rotor is thereby driven in rotation to produce rotary motion and transmit mechanical power via an output shaft which may be coupled to a driven load.
In conventional electric motors the rotor and output shaft are most often supported by anti-friction bearings on either end of the motor housing. Depending upon the size of the motor and the anticipated loading, the rotor and shaft support bearings may be journal bearings, needle bearings, roller bearings, ball bearings, and so forth. To prolong the useful life of the rotor bearings, it is commonplace to equip the motor with a means for lubricating the bearing elements during operation. For example, the bearings may be provided with a synthetic or mineral grease or oil which coats surfaces of the bearing elements and which may preclude the ingress of contaminants, such as dirt, debris, moisture, and so forth into the bearing. In other applications, a pressurized oil mist may be circulated through a bearing cavity to provide continuous lubrication of the bearing and to similarly prevent the ingress of contaminants, while flushing the bearing cavity of contaminants and moisture.
A difficulty which arises in oil mist-lubricated motor bearings involves the escape of oil mist into the motor housing and outwardly into the atmosphere surrounding the motor. In particular, in totally-enclosed, fan-cooled motors, a negative pressure may be created within the motor housing due to the circulation of air through the housing under the influence of a fan mounted on the rotor shaft. In many applications no seal is used to retain the oil mist within the bearing cavity. Even where seals are employed these are typically provided for preventing the ingress of contaminants and moisture into the bearing cavity, often inadvertently facilitating the conveyance of oil mist from the bearing cavity into the motor enclosure.
Other difficulties in sealing anti-friction bearings installed on electric motor shafts arise from the relative speeds between the sealing elements and the rotating elements, typically the shaft itself. In particular, while certain sealing arrangements may be employed to isolate inner bearing cavities from ambient air and from the inner volume of the electric motor housings, the use of such seals is typically limited by the relative surface velocity of the rotating and non-rotating elements. In particular, seals designed to ride on rotating shafts or other rotating elements secured to the shafts may require certain relative velocities in order to maintain an adequate seal. In certain applications, the motor shaft must be increased in size to provide sufficient velocity to maintain the desired seal, despite the fact that the oversized dimensions of the shaft are not necessary for transmitting anticipated torque levels. However, where such velocities become excessive, the seal life may be significantly reduced due to heating caused by friction between the seal and the rotating member. Moreover, depending upon the motor design and its rated speed, the same sealing arrangements may not be applicable due to the increased speed of the motor (e.g., two-pole speeds of 3600 rpm in 60 Hertz electrical supply systems).
There is a need, therefore, for an improved technique for providing a seal adjacent to a motor bearing which is effective at maintaining an oil mist or similar lubricant within the environment of the bearing, while also inhibiting the ingress of contamination and moisture into the vicinity of the bearing. In particular, there is a need for a sealing technique for motor bearings which can be both installed on new motors as well as retrofitted on existing motors in which such lubricant systems are employed. There is also a particular need for a sealing technique for motor bearings which provides an enhanced sealing capability despite the particular speed of the rotor shaft, and that avoids or reduces the tendency of the shaft seal to overheat due to high relative surface speeds during operation.